This invention relates to apparatus for receiving ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system such as a satellite positioning system, for example, the Global Positioning System (GPS).
The original concept of GPS envisaged user equipment with an antenna having a largely clear view of the sky, whether mounted on aircraft, ships, land vehicles or in portable applications. Increasingly, however, there are requirements for GPS receivers capable of operating in poor signal conditions and which operate at higher than previously accepted accuracies. Weak signal conditions in ranging systems such as the GPS satellite-based system, are usually caused by obstructions or multipath propagation. Such reception conditions often occur inside buildings, but also outdoors in densely built-up areas or under foliage. The region over which a GPS receiver can be expected to operate can be improved by maximising the received power-to-noise ratio for each satellite signal.
Signal reception of GPS inside buildings is characterised by a transmission model that has no dominant mode since most signals arrive at the receiver after one or more reflections. This transmission model implies that the signal has Rayleigh fading statistics. Such statistics are generally found when there are multiple signal paths to the receiver, each model allows the Central Limit Theorem to be invoked to describe signal amplitude. The magnitude of the resulting signal, free of signal phase measurements, may then be established as a two-dimensional normally distributed random variable. The magnitude is a first order "khgr"2 distribution with the characteristic Rayleigh probability density function:
P(R)=(R/"sgr"2)exp(xe2x88x92R/2"sgr"2)
The direction of the signal arrival is not well defined in this transmission model and may come from any direction. In reality, this will be constrained by physical factors such as nearby windows capable of GPS signal transmission. For the most part, it will also not be possible to determine the polarisation of the signal. GPS satellite transmissions are right-hand circularly polarised but become circularly or elliptically polarised on reflection from metallic objects.
In the circumstances described above, the signal field can be subject to deep fades, similar to those experienced by FM radio receivers in automotive environments.
Another difficulty is the shrinking physical size of GPS receivers bringing, amongst other problems, the danger of self-jamming due to electromagnetic noise from small devices with large gate counts exceeding local thermal noise levels. In addition, the external electromagnetic environment is deteriorating as a result of increasing congestion in the electromagnetic spectrum. As an example, digital broadcast television, which is due to expand rapidly in coming years, has relatively high out-of-band emissions compared with previous systems. Other satellite communication services are being allocated spectrum space in bands adjacent GPS frequencies.
This invention aims to counter such difficulties by providing a receiver system having a plurality of antennas. According to a first aspect of the invention, apparatus arranged to receive ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system comprises a radio receiver unit having a plurality of radio frequency (r.f.) input connections, and a plurality of spaced-apart antennas each connected to a respective said input connection, wherein the receiver unit includes: an antenna signal processing circuit responsive to the ranging signals for producing output signals including components each individually representative of at least one characteristic of the ranging signals received at a respective antenna; and a signal combiner configured to receive the output signals, to monitor, for each of a plurality of the said remote transmitters, at least one characteristic of the ranging signals from that remote transmitter via the different antennas, and to weight one or more of the said components to yield a range estimate.
According to a second aspect of the invention, the apparatus has a receiver unit with a signal selection arrangement configured to receive the output signals, to monitor, for each of a plurality of the said remote transmitters, at least one characteristic of the ranging signals from that remote transmitter via the different antennas, and to select one or more of the said components to yield a range estimate.
The signal combiner or selection arrangement is preferably configured to weight or select the components according to the signal-to-noise ratios of the ranging signals received from at least one of the transmitters, advantageously from each of them. Alternatively, or in addition, the signal combiner or selection arrangement may be configured to weight or select the components according to the amplitudes of the ranging signals whether from one or several of the transmitters.
Typically, the antenna signal processing circuit includes at least one code-tracking loop for tracking code modulation of the ranging signals when received by the antennas. The selection may be performed periodically so as to select one of the ranging signals the amplitude of which is greater than the others, and then to apply the selected signal to the code tracking loop. There may be a number of code tracking loops, the signal selection arrangement being configured periodically to select a number of the ranging signals by weighting the magnitudes according to a predetermined weighting scheme which is dynamically variable, being adapted to the changing electromagnetic conditions as happens, for instance, when a motor vehicle containing the apparatus passes through a built-up area.
In the last-mentioned situation, the changing electromagnetic environment is largely due to varying intensities of signals reflected from adjacent buildings. Thus, the signal combiner or selection arrangement may be specifically configured to weight or select the components due to signals from the different antennas according to their multi-path content.
As an alternative to using a code-tracking loop for tracking code modulation of the ranging signals, a parallel correlator structure may be used.
Although, in the case of GPS, code-tracking may be relied upon to provide position fixes, improved accuracy may be obtained by measuring carrier phase. In the context of the present invention, a receiver unit including a carrier phase tracking loop may include a phase-shifting arrangement operable to apply a selected phase shift to a selected or weighted signal, the magnitude of the phase shift being dynamically adjusted according to the antenna via which the selected or weighted signal is received. Accordingly, it is possible to configure the phase-shifting arrangement to adjust the phase shift in a manner that maintains coherent phase matching between ranging signals received via the different antennas to improve phase tracking accuracy. Coherent phase tracking may be maintained in order to allow switching between different selected ranging signals from different antennas.
Phase and amplitude taper multiplication coefficients may be applied to the ranging signals from the different antennas using, for instance, an adaptive equaliser. Phase taper, then, can cause the ranging signals received at different antenna elements from a particular direction to reinforce each other, giving a means of selecting signals from a particular direction (e.g. from a particular transmitter), and for rejecting signals received from other directions, e.g. due to unwanted reflections or other sources of interference. An amplitude taper can act as a spatial filter altering the combined beam pattern of the antennas as an array.
According to a third aspect of the invention, apparatus arranged to receive ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system comprises a radio receiver unit having a plurality of r.f. input channels and a plurality of spaced-apart antennas each connected to a respective said input channel, wherein: the receiver unit includes at least one antenna signal processing circuit responsive to the carrier phases of the ranging signals received via at least one of the antennas; each input channel i includes a multiplying element arranged to combine a respective first multiplier signal Mi with an incoming signal in the channel, the multiplier signals constituting an orthogonal set of signals; and the receiver unit further includes a combining element for combining the multiplier output signals resulting from combining the incoming signals with the multiplier signals, a common intermediate frequency channel including common filtering means for receiving the multiplier output signals from the combining element, an analogue-to-digital converter (ADC) coupled to an output of the common channel, and a processor for applying second multiplier signals to a digitised signal obtained from the ADC thereby substantially to isolate from each other signals representative of respective incoming signals from respective antennas, each second multiplier signal corresponding to or being the same as the first multiplier signal for the respective antenna; whereby phase coherence is maintained between the different signals associated with the different antennas.
In such apparatus, as well as apparatus as described above in accordance with the first and second aspects of the invention, the antennas may have different polarisation characteristics, the processor being programmed to combine digitised representations of the different antenna signals to weight or select received signals having a predetermined polarisation. The antennas may comprise a pair of antennas each formed as a linear element, the linear elements being arranged perpendicularly to each other so that, according to the way in which the digitised representations of the signals are combined, the receiver unit selects received signals of right-hand circular polarisation or left-hand circular polarisation. By arranging for the processor to be programmed to weight and combine the digitised representations of the different antenna signals, it is possible to select received signals of elliptical polarisation. In such cases, the processor is preferably configured to implement sum and difference networks. The digitised representations may be combined variably, e.g. in an adaptively varying manner in order to optimise signal reception in a multipath environment.
The processor may be programmed, in addition, to apply selected weightings to the signals isolated after passing through a common channel in the receiver unit in order to tailor the response of the receiver unit to favour received signals having a predetermined characteristic. Such weightings may be adaptively variable.
In preferred apparatus having a common channel for signals representative of incoming signals from a plurality of different antennas, the receiver unit may have input channels which include a pair of sub-channels for I and Q components of each respective incoming signal, the processor being programmed to combine the I and Q components after the isolation step.
CDMA techniques may be used in the receiver unit to combine signals received from individual transmitters at different antennas in order to feed them through a common channel which maintains phase coherence and which, in particular, can filter signals with a constant group delay and, then, subsequently to separate them for further processing. Accordingly, the receiver unit may include a multiplying combiner stage for each channel which multiplies the incoming signal from each antenna by a spreading code assigned to that antenna. The signals may then be combined by adding and feeding them through the common channel. Corresponding despreading codes may be applied at the end of the common channel in order to separate the signals representative of the incoming signals from different antennas. The spreading and despreading codes may be signals constituting an orthogonal set, as mentioned above.
It will be appreciated that much of the receiver architecture may be implemented in software. There is also a choice as to the point in the receiver chain at which analogue signals are converted to digital form. Thereafter, processing may take place by hardwired logic or software processing. It will also be appreciated that the references to channels above include virtual channels whereby circuitry is shared between channels on a multiplexed basis or where different channels are formed in the software routines.
According to a further aspect of the invention, apparatus arranged to receive ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system comprises a radio receiver unit having a plurality of radio-frequency (r.f.) input channels, and a plurality of spaced-apart antennas each connected to a respective said input channel, wherein the receiver unit includes an arrangement for coherently combining and processing signals representative of the ranging signals received by the antennas from each of a plurality of the transmitters, the arrangement advantageously includes a common signal processing channel including common filtering means to maintain phase coherence between output signals associated with any one of said transmitters, being signals extracted from the common channel and representative of the signals received at the antennas from that transmitter. Such apparatus can allow the individual adaptation of antenna array sensitivity pattern for each of the plurality of transmitters, irrespective of the line-of-sight directions of the other transmitter as viewed from the antennas.
Ranging signals may be received from remote transmitters in the form of earth-orbiting satellites, as in a GPS embodiment. Terrestrial systems are also possible, as are combined systems in which one or more of the transmitters is an earth-orbiting satellite, and one or more others are so-called xe2x80x9cpseudolitesxe2x80x9d being transmitters which, in a GPS embodiment, transmits signals in the same format and at the same frequency as satellite-based transmitters.
The extent to which reception quality can be improved is influenced by the spacing of the antennas. In most embodiments, the antenna elements are much closer together than the GPS code phase wavelength (300 meters). However, the ranging signal wavelength at GPS is 19 cms for the L1 transmissions. It follows that carrier phase outputs from different antennas spaced by distances of the same order will be different, giving the ability to weight and select signals for improved reception, as described above. However, the invention is not limited to apparatus having such antenna spacings. Advantages can be gained by placing the antennas with a spacing approaching the maximum possible afforded by the platform in question, such as at the front and rear of a motor vehicle. Advantages can be gained by having several antennas forming an array in which the inter-antenna spacing between neighbouring antennas is less than xcex/2 where xcex is the wavelength of the ranging signals in air, but with the maximum span of the array greater than 5xcex. In the general case, the antennas are spaced apart by 50 mm or more to provide signals whose magnitudes are to some degree statistically independent.
Other aspects of the invention include receiving ranging signals via at least two antennas for independently sampling the local signal field, and employing a simple algorithm to select the largest signal from the two antennas, or to favour receiving direction to select transmitters having independent fading characteristics. Processing of the signals from each satellite is performed separately in choosing the selection or weighting of the signals. Processing may be performed incoherently or coherently, the advantage of coherent processing being that it allows use of carrier phase differences between the signals received from individual transmitters at the different antennas to be employed in improving reception. Thus, application of a phase shift to one of the antennas may by used align its phase with that of the signal received at one of the other antennas. A steering function may be applied in this way to the antenna signals with the maximum gain pointing in the direction of arrival of the required signal. This phase taper technique allows a 3 dB improvement in the signal-to-noise ratio using two antennas.
As mentioned, above, both amplitude and phase taper may be used to maximise the signal extracted from the processor stages in the receiver unit. Both amplitude and phase taper functions may be changed dynamically to maintain the GPS signal from a chosen satellite at its maximum value, the amplitude and phase taper being adapted to the signal conditions.
One other feature of the preferred embodiment of the present invention is the use of CDMA to pass GPS signals continuously through common parts of the receiver. This has the advantage of improving the signal-to-noise ratio of each of the satellite signals being observed. Furthermore, there is no requirement to select a master GPS signal; all signals have equal importance.
One of the aspects of the invention referred to above is a special arrangement of antennas allowing discrimination between signals of different polarisation. Typically, the linear elements of the antenna referred to above consist of a pair of dipoles arranged in crossed format. Rather than connecting the elements to a coupling network to form a combined antenna sensitive to signal fields with either left-hand or right-hand circular polarisation, in the present preferred embodiment, both antenna signals pass through the receiver unit and associated correlator structure phase coherently. Subsequent digital signal processing is arranged to sum the two antenna signals with either a plus 90xc2x0 or minus 90xc2x0 relative phase shift with the consequence that the receiver unit can be made to choose either left-hand or right-hand circular signal polarisation. The alternative of making the system sensitive to elliptical polarisation, as referred to above allows any form of such polarisation to be accommodated by suitable choice of the gain relationship between the channels.